Characterization of the High-Strength Mg-3Nd-0.5Zn Alloy Prepared by Thermomechanical Processing

doi:10.1007/s40195-018-0765-x

Abstract

Abstract:

Magnesium alloys based on Nd and Zn are promising materials for both aviation industry and medical applications. Superior mechanical properties of these materials can be achieved by thermomechanical processing such as extrusion or rolling and by aging treatment, which can significantly strengthen the alloy. The question remains especially about the connection of texture strength created in the alloys based on the specific conditions of preparation. This work focuses on the Mg-3Nd-0.5Zn magnesium alloy prepared by hot extrusion of the as-cast state at two different temperatures combined with heat pre-treatment. Extrusion ratio of 16 and rate of 0.2 mm/s at 350 and 400 °C were selected for material preparation. The structures of prepared materials were studied by scanning electron microscopy and transmission electron microscopy. The effect of microstructure on mechanical properties was evaluated. Obtained results revealed the strong effect of thermal pre-treatment on final microstructure and mechanical properties of extruded materials. The Hall-Petch relation between grain size and tensile yield strength has been suggested in this paper based on the literature review and presented data. The observed behavior strongly supports the fact that the Hall-Petch of extruded Mg-3Nd-0.5Zn alloys with different texture intensities cannot be clearly estimated and predicted. In addition, Hall-Petch relations presented in literature can be sufficiently obtained only for fraction of the Mg-3Nd-0.5Zn alloys.

Key words: Mg-Nd-Zn alloys, Extrusion, Thermal treatment, Microstructure, Mechanical properties

Ji$\check{r}$í Kubásek, Drahomír Dvorsky, Jozef Vesely, Peter Minárik, Mária Zemková, Dalibor Vojt$\check{e}$ch. Characterization of the High-Strength Mg-3Nd-0.5Zn Alloy Prepared by Thermomechanical Processing[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(3): 321-331.

Figures/Tables 10

Table 1 Designation of studied materials in relation to the processing conditions

Label	AC+Ex350	T4+Ex350	AC+Ex400	T4+Ex400
Process	As-cast ingot extruded at 350 °C	Solution-treated ingot extruded at 350 °C	As-cast ingot extruded at 400 °C	Solution-treated ingot extruded at 400 °C

Fig. 1 Microstructure of Mg-3Nd-0.5Zn alloys (SEM): a AC+Ex350, b T4+Ex350, c AC+Ex400, d T4+Ex400. Light particles and dark areas correspond to intermetallic phases especially Mg41Nd5 and primary α-Mg, respectively

Fig. 2 Intermetallic phases observed in extruded Mg-3Nd-0.5Zn (TEM): a, b AC+Ex350, c T4+Ex350. Individual figures illustrate the presence of specific phases with their corresponding SAED patterns. ZA designation means zone axis

Fig. 3 Intermetallic phases observed in extruded Mg-3Nd-0.5Zn (TEM): a AC+Ex400, b T4+Ex400. Individual figures illustrate the presence of specific phases with their corresponding SAED patterns. ZA designation means zone axis

Fig. 4 EBSD maps and inverse pole figures of studied Mg-3Nd-0.5Zn alloy: a AC+Ex350, b T4+Ex350. White lines in EBSD maps represent low-angle grain boundaries characterized by angle up to 15°. X0 corresponds to the direction of extrusion. Inverse pole figures reflect continuous orientation distribution with contour levels showing the strength of the texture as the number of times random occurrence

Fig. 5 EBSD maps and inverse pole figures of studied Mg-3Nd-0.5Zn alloy: a AC+Ex400, b T4+Ex400. White lines in EBSD maps represent low-angle grain boundaries characterized by angle up to 15°. X0 corresponds to the direction of extrusion. Inverse pole figures reflect continuous orientation distribution with contour levels showing the strength of the texture as the number of times random occurrence

Fig. 6 Distribution of grain size in Mg-3Nd-0.5Zn alloy processed by extrusion at different conditions

Table 2 Mechanical properties of studied materials

Sample	CYS (MPa)	UCS (MPa)	D (%)	TYS (MPa)	UTS (MPa)	E (%)	HV1
AC+Ex350	224 ± 6	450 ± 11	14.8 ± 0.5	337 ± 6	338 ± 6	0.5 ± 0.2	64.1 ± 1.8
T4+Ex350	225 ± 1	477 ± 18	13.2 ± 0.3	384 ± 4	385 ± 3	2.8 ± 0.2	67.6 ± 1.9
AC+Ex400	174 ± 5	423 ± 12	12.7 ± 0.4	251 ± 6	271 ± 8	5.9 ± 1.0	61.1 ± 2.3
T4+Ex400	172 ± 2	424 ± 13	11.8 ± 0.4	312 ± 9	321 ± 11	2.7 ± 0.7	61.3 ± 2.6

Fig. 7 Tensile (dash line) and compressive (solid line) stress-strain curves obtained for Mg-3Nd-0.5Zn alloy at room temperature and a strain rate of 0.001 s-1

Fig. 8 Dependence of tensile yield strength on inverse square root of average grain size (Hall-Petch relation) for Mg-3Nd-0.5Zn alloys [1, 4, 9, 14, 18, 19, 20, 21, 26, 38, 39]. AC = as-cast; T4 = 540 °C/10 h+water quenching; T6 = T4+200 °C/14 h; Ex = AC/T4+Extrusion (T = 300 °C, 320 °C), ER (extrusion ratio) = 8, 9, 25; Ex+aging = Extrusion+200 °C/8 h

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